Longwall mining is a highly productive type of mining requiring a highly specialized machine. The mining environment for using this longwall machine must be prepared before machine installation can be carried out. A block of coal (or other minable material), called a "panel," is established by defining the panel perimeter within the material seam by tunnelling. Typically, the panel is 500 to 1,000 feet across and 5,000 feet in length. Tunnels along the length of the panels are used for entrance by personnel and equipment and are used for material removal after extraction by the longwall shearing machine. The shearing machine is established at one end of the panel, and a track is build along the width for it to traverse. The track is constructed by joining sections together to form the basis for the machine travel path and to construct the face conveyor system onto. The sections are typically called "pans", and the pans are typically about five feet in length. The track, shearer machine, and personnel are protected from cave-ins by a series of roof support shields which are also typically about five feet in length. The shearing machine is constructed to have one or more ranging arms with material extraction devices on the end(s). The ranging arms are pivoted at one point, and the arm is rotated about this point for the purpose of positioning the extraction end of a desirable height for material extraction or clearance. The extraction operation is performed by the shearing machine traversing the width of the panel, cutting the material from the seam wall, called the "face," and the material being removed by a face conveyor system. The machine travels in both directions and, as one full cut is completed, the roof support shields, conveyor, and machine are advanced into the face for the next cutting sequence. This material extraction and advancement of the equipment will continue until the panel is completed. Coal seams mined in this manner are usually in the range of 48 to 144 inches in vertical thickness, with an adjacent region above and below a seam of varied composition. Frequently, the adjacent region is of clay or other unstable material, and for this reason, the tunnel in which the cutting operation occurs is often formed in the coal to maintain the integrity of the tunnel above and below the cutting machine during its pass through the tunnel. As conditions may require, from two to six inches of coal may be left on the ceiling and/or floor. For example, sufficient thickness of coal on the floor is necessary to support the weight of the shearing machine and roof supports in areas where soft floor materials are prevalent. Alternately, if a greater amount of coal is left than actually required, the economic impact is substantial and can run into the millions of dollars if such deviations persist during significant mining operations. In addition, automated positioning of the cutting drums can result in smoother cuts with less steps, etc., which reduces equipment breakage and maintenance. Also, operating personnel can move away from the cutters, reducing their exposure to coal dust and flying debris.
The obvious problem is how to accurately track and follow a coal seam and its boundaries. If coal seams were level, the task would be relatively simple, but they are not. Instead, they largely tend to follow a constantly changing slope and, in fact, in many instances, conform to sine waves with amplitudes typically running from 3 to 12 feet and periods of typically 50 to 100 feet. Another and related factor is that the cutting or shearing drums of the cutting machine extend significantly beyond the ends of the machine and cut to the side of the machine and thus form a tunnel for the next pass by the machine. The problems that arise will be appreciated from a brief examination of operation.
Assume first that an initial tunnel is precisely cut between access tunnels along a seam and that it has a known sine wave contour. A track, made of sections generally referred to as pans, is laid through the tunnel beside a face of the seam to be sheared. The shearing machine is then positioned on the track, and its initial task is to make a first cut in the face of the seam, beside the initial tunnel. Ideally, a cutting drum would be set at one elevation to cut either the floor or ceiling of a new cut which is identical with the ceiling or floor elevation of the initial tunnel. Unfortunately, while the machine body is typically supported by its four rollers or support slides in the initial tunnel, a cutting drum is in advance of the body of the machine by several feet as it cuts, in effect, a new tunnel. The result is that the elevation of the body is in terms of the slope of the tunnel under it, whereas the cut made by the cutter, to the side of the initial cut, is in advance of the body of the machine. As a result, a single relative position of the drum with respect to the machine will not produce side-by-side like contoured cuts. If one does proceed in a single position of the drum, there will be produced a new cut which will follow a sine wave, but this sine wave will be longitudinally displaced with respect to the sine wave contour of the original tunnel, and it thus will not conform to the coal seam. To make matters worse, actual measurement of the position of the drum with respect to the floor is not possible because of a too-hostile environment for instrumentation.
Because of these factors, it was necessary to devise systems for automated control of a longwall shearer. One such system (which is incorporated herein) is disclosed in U.S. Pat. No. 4,634,186, entitled "Control System For Longwall Shearer," issued to Robert E. Pease on Jan. 6, 1987. In this patent, a series of coal thickness sensors are employed, as in the pan track assemblies upon which the shearer rides. During the period that the track is positioned in one position for a face cut, for example, 15 to 20 minutes, these sensors each determine the thickness of the coal at their location. For example, they may be 20 to 50 feet apart. Each reading is compared with a selected thickness and a difference quantity, indicating, by one sign, less thickness than desired, or by the other sign, a greater thickness than desired; or, if the comparison provides a zero, then the ideal thickness obtains. The difference figures for the series are typically arranged in some format to determine a smooth curve of cut corrections along the track and these placed in a memory. Then, as the machine traverses the track during the next path of the shearer, corrections are taken from the memory corresponding to measured locations and added to or subtracted from the elevation of the cutter at that location. This provides an extremely precise control of coal thicknesses left at the ceilings and floors of the coal. At the same time, or if no coal thickness sensors are employed, manual offsets may be entered to readjust the floor or roof elevation if required at discrete locations on subsequent passes.
The applicant has provided a control system which includes inclinometers mounted in predetermined angular relation on the ranging arms to provide instantaneous output signals indicative of the roll and pitch attitude of the cutting drum. Such output signals are used to control the angle at which the cutting drums engage the face at subsequent passes of the drums across the face.